In such a machine, as described for example in commonly owned U.S. Pat. No. 3,117,348 in the name of Herbert Rees, the originally granular molding material is liquefied and compacted by the plasticizing screw, moving in a heated barrel, and is transferred to a precompression chamber from which it is expelled into a sprue channel either by the screw itself or by an associated injection piston. If the screw also performs the injection function, its rotation must be stopped during the forward stroke; this is inconvenient because it reduces the efficiency of the screw as a plasticizing device and because the repeated starting and stopping subjects the mechanism to considerable wear.
When an injection piston is used, the continuously rotating screw is periodically repressed by the plastic mass accumulating at the forward end of its barrel while its outlet is blocked by a valve before and during an injection stroke. This repression takes place against a restoring force which advances the screw upon the unblocking of the outlet and the retraction of the injection piston from the precompression chamber which is thereupon promptly refilled at the start of a new cycle. The restoring force should not be constant but should progressively increase as the screw moves back so as gradually to build up the compression of the plasticized mass to a predetermined level at the end of the repression movement, i.e. just prior to the unblocking of the outlet; with the screw rotating at constant speed, its backward motion slows down with increasing compaction of the mass whereby its stroke automatically adjusts itself to that of the injection piston. Such a restoring force conforms to the compression characteristic of a coil spring, as described in the aforementioned Rees patents, yet the use of springs is practical only with relatively short strokes.